So, I have an Electrolux tumble dryer, Electrolux attachment for in between and want to get a Miele washing machine.

Should I use Miele or Electrolux attachment between these two?

(I know that having both same brand is the best but don't want to pay for an extra tumble dryer)

I had this in a previous place (not those brands, but two different ones) and the bracket was for the top one because it aligned with the feet etc, whereas the bottom one just has a flat top.

I seem to recall it required some "adjustment" (i.e. sawing off the back rim since the lower unit was longer), but it was fine as the main security was lots of sticky foam tape holding onto the top of the base.

You can buy universal ones which are adjustable.

If you can't find a universal attachment that is suitable then you need to get the base attachment suitable for the top machine.

The top of the machine underneath will generally be made of plastic-covered chip-board and you can mount the joining bracket on here securely with screws of a correct length (marking and drilling pilot holes first) and then slide the top machine into place into the bracket and securing.

That's what I did last time.

My experience is opposite to what others have suggested.

We experienced a problem when Miele replaced our 6 year old Miele washing machine with a new model but the Miele dryer was not replaced. The newer Miele washer had a different shape than the older model. The old attachment kit would not work with the new washer even though it was fine for the old dryer.

The Miele installer used a new attachment kit which he had to modify so that the dryer feet would sit securely in place. It looked a bit odd but it was secure.

So maybe keep the existing attachment piece from the current washer and hope it fits on the new Miele.

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I suppose the OP could modify a bracket too - that may be a solution but probably easier when both the washer and dryer are from the same manufacturer - which, and this is the important point, is the not what the OP will have to deal with.